A 900MHz quarter wave antenna is electrically connected to the Duplexer through a low-loss coax cable. The minimum distance between the Telecomm antenna and other antenna for equipment operating in the 800 to 1200 MHz including DME, Transponder, and TCAS should be at least three feet. In the event multiple ARTUs are used, the antenna spacing is at least four feet between like units, and a minimum of two feet should be observed from all other antennas. If the antenna is to be installed on a composite skin, a metal surface extending one foot radially from the antenna connector is required to provide a ground plane. There should be less than 2.5 milliohms of resistance between the antenna and the airframe. Corrosion or paint under the antenna can result in a significant signal loss. Problems with the various co-ax cables and cable connectors are other common causes of difficulty with airborne telecommunication systems.
The Local Area Network (LAN) within the aircraft can consist of one or more telephone handsets, fax machine, and portable computer connections. A System Interface Unit (SIU) used by the Claircom system, or a Cabin Distribution Bus Repeater (CDBR) used by MagnaStar, are the components used to connect all the user equipment to the ARTU. This will include the cabin and flight deck handsets, fax machine, computer modems, and even includes SATCOM or HF Radios. In addition, a CDBR provides select calling to a specific telephone handset in the aircraft as well as managing available phone lines and connection to the usually more expensive SATCOM when all Air to Ground lines are in use or unavailable.
Most handsets in use today are digital devices and are used like a traditional telephone. They include a microphone and a small speaker, a keypad, and in some cases, a jack to allow connection of an external modem. The keypad may also be used as a device to program the entire communication system, but usually requires some type of password to get beyond all but the basic keypad functions. It is important to refer to the specific system user"s manual prior to entering the programming functions.
The means of operation of Air to Ground communications is much like a pager system. When someone on the ground wishes to contact a person in an aircraft, the appropriate telephone number is entered and a corresponding code is broadcast by all of the specific service provider"s ground stations. After the signal is received by the airborne receiver transmitter, an analysis is made as to which ground cell is providing the best signal. The ARTU will then call back to the best signal provider. It is this call back that initiates the communication link. As the aircraft continues on its course, the signal from the in-use ground station will begin to deteriorate. Another function of the ARTU is to listen to all available ground stations and make a decision based on signal condition as to which cell to handoff the phone call. This decision is based on aircraft direction and speed as well as ground station location and proper function. The handoff, when it occurs, should not be noticed by the system consumers.
In addition to voice communication, facsimile and PC modems are common users of in-flight telephone systems. Transfer of electronic data provides some interesting challenges. First of all, computer modems have gotten faster. Unfortunately, some in-flight telecommunication systems may not keep up. Presently, the data transfer or baud rate can be from 2,400 Bits Per Second (BPS) to 19,200 BPS. Also, when the initial "handshake" is made between the two communicating computers, the modems have to synchronize. As the aircraft system is limited, the ground system has to adjust its speed. This has to occur before the connection timeout, otherwise disconnection is imminent.
The most frequent culprit causing failure of in-flight data communications is from improperly matched modems. It is always best to contact the system manufacturer and determine modem compatibility prior to sending the aircraft on an extended mission with a group of executives who otherwise will be unable to retrieve their e-mail. The second most common problem is lack of knowledge on the part of the passengers as to proper operation of the system. In the event trouble is reported, it is important to find out if the problem was encountered by only one passenger or by all passengers. If only one person reports improper operation, the probability is high that the trouble is with the user and not the system.
Another important question to ask in the event of reported problem is aircraft location and if the voice communication was operational. Lack of voice communication accompanying loss of data communication could point to a system malfunction or even an antenna problem. The question of location might be important if the aircraft had traveled out of the service area or had managed to cross one of the few areas out of the range of a ground station.
By reducing antenna size and increasing affordability, this new system will make satellite-based voice and data connectivity practical for a whole new segment of the industry.
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